This invention relates to a method and apparatus for actively varying or modulating the operating frequency of a pulsed detonation combustor in order to avoid exciting natural resonant frequencies of an engine system.
In many applications, and especially in aircraft engines, excessive forced response in propulsion or power conversion equipment is avoided by resonant frequency avoidance. Namely, the driving frequency from some driving force or combustion source is purposely chosen to not coincide with a structural resonance or acoustic resonance, of the surrounding structure. In most cases, the final design of structure or hardware contains a fixed set of natural resonant frequencies. In the design process, one of the objectives is to make sure that these natural resonant frequencies do not coincide with any driving power source frequencies.
In the case of reciprocating engines, axial or centrifugal flow propulsion engines and axial or centrifugal flow power conversion engines the forcing frequency is a function of engine speed. Thus, designers typically avoid operating at speeds where the forcing function and natural frequencies coincide.
With the advent of the use of pulse detonation devices in propulsion applications, particularly aircraft propulsion, this consideration must also be addressed as pulse detonation devices create an impulsive load to the chamber and adjacent components. This load can excite resonant modes in structures. Additionally, fixing the frequency of pulse detonation devices can provide additional coupling to structural resonant tones such that excitation exceeds component damping leading to increased or accelerated component fatigue and/or failure of components and structure.
Up until the advent of the present invention, this problem has not been addressed with pulsed detonation or quasi-detonation combustion sources. For traditional power sources, firing frequencies are fixed to engine operating speeds and structures are chosen to avoid resonance coupling.